1. Field of the Invention
This invention relates to a driving force control system for a four-wheel drive vehicle including a pair of front wheels, and a pair of rear wheels, one of the pairs being main drive wheels, and another of the pairs being auxiliary drive wheels, the driving force control system controlling engagement forces of clutches to thereby control driving forces distributed to the auxiliary drive wheels.
2. Description of the Prior Art
This kind of driving force control system has been proposed by the present assignee, e.g. in Japanese Patent Publication (Kokai) No. 10-194005. The four-wheel drive vehicle (hereinafter simply referred to as xe2x80x9cthe vehiclexe2x80x9d) is provided with left and right electromagnetic clutches for connecting and disconnecting a propeller shaft to and from left and right rear wheels. The vehicle uses front wheels as main drive wheels and the rear wheels as auxiliary drive wheels. Further, the vehicle is provided with a lock switch. The driving force control system controls engagement forces of the respective left and right electromagnetic clutches to thereby control torques to be distributed to the rear wheels, i.e. the auxiliary drive wheels. More specifically, this system operates in an automatic mode for controlling the engagement forces of the left and right electromagnetic clutches in response to signals from various sensors, and in a lock mode for controlling the engagement forces of the clutches to a maximum engagement force which can lock the front wheels and the rear wheels to each other. The lock mode is executed e.g. when the driver operates a lock switch for the purpose of extricating the vehicle from a stuck condition on a snowy road. In the lock mode, the maximum amounts of control currents are supplied to the respective left and right electromagnetic clutches so as to control the engagement forces thereof to the maximum.
However, according to the above conventional driving force control system, after the vehicle gets unstuck from a stuck condition, the vehicle can sometimes enter a traffic-congested traveling condition in which the running of the vehicle is controlled by engine brake or foot brake, with the accelerator pedal being scarcely stepped on by the user, with its lock switch being kept ON. In such a case, since the lock mode continues to be executed, the maximum amounts of control currents continue to be supplied to the respective left and right electromagnetic clutches in spite of the fact that it is almost unnecessary to distribute the driving force delivered to the front wheels to the rear wheels, which results in waste of electric power. Further, since the engagement forces of the respective electromagnetic clutches are each controlled to the maximum engagement force, the fuel economy of the engine is degraded.
It is an object of the invention to provide a driving force control system for a four-wheel drive vehicle, which is capable of properly controlling the engagement forces of clutches such that the engagement forces are neither excessive nor short, in dependence on operating conditions of the vehicle while complying the driver""s intention as much as possible, thereby making it possible to reduce the sizes of the clutches and the driving system and at the same time improve the fuel economy.
To attain the above object, the present invention provides a driving force control system for a four-wheel drive vehicle including a pair of front wheels, and a pair of rear wheels, one of the pairs being main drive wheels, and another of the pairs being auxiliary drive wheels, the driving force control system controlling engagement forces of clutches to thereby control driving forces distributed to the auxiliary drive wheels.
The driving force control system according to the invention is characterized by comprising:
automatic-mode engagement force-calculating means for calculating automatic-mode engagement forces of the clutches based on operating conditions of the vehicle;
input means for being operated by a driver;
lock mode execution means for executing a lock mode in which the engagement forces of the clutches are set to a lock-mode engagement force, including a lockable engagement force which can lock the main drive wheels and the auxiliary drive wheels to each other, when lock mode-executing conditions including operation of the input means by the driver are satisfied;
lock-mode engagement force-limiting means for limiting the lock-mode engagement force such that the lock-mode engagement force is held below the lockable engagement force in dependence on a traveling condition of the vehicle; and
clutch engagement force-selecting means for selecting the calculated automatic-mode engagement forces as the engagement forces of the clutches, when the calculated automatic-mode engagement forces are larger than the limited lock-mode engagement force, during execution of the lock mode.
According to this driving force control system, the automatic-mode engagement forces are calculated based on the operating conditions of the four-wheel drive vehicle. Further, when the lock mode-executing conditions including operation of the input means by the driver are satisfied, a lock mode is executed in which the engagement forces of the clutches are set to a lock-mode engagement force, including a lockable engagement force which can lock the main drive wheels and the auxiliary drive wheels to each other. As a result, during the lock mode, it is possible to lock the main drive wheels and the auxiliary drive wheels to each other, in a manner complying with the driver""s intention as much as possible. Further, the lock-mode engagement force is limited depending on a traveling condition of the vehicle, such that it is held below the lockable engagement force. As a result, when there is little necessity to distribute the driving forces to the auxiliary drive wheels, e.g. when the vehicle is in a traffic-congested traveling condition in which the running of the vehicle is controlled by engine brake or foot brake, with the accelerator pedal being scarcely stepped on, it is possible to limit the engagement forces of the clutches in the lock mode to a smaller value than the maximum engagement force according to the degree of necessity of distribution of the driving forces to the auxiliary drive wheels, thereby minimizing time during which the engagement forces of the clutches are controlled to the maximum. This not only makes it possible to save energy for driving the clutches, but also to reduce the sizes of clutches and the driving system and improve the fuel economy of the engine. Moreover, when the calculated automatic-mode engagement forces are larger than the limited lock-mode engagement force, the engagement forces of the clutches are set to the automatic-mode engagement force. As a result, it is possible to comply with the driver""s intention as much as possible and at the same time distribute the driving force actually required by the four-wheel drive vehicle to the auxiliary drive wheels, such that the engagement forces of the clutches are controlled to be neither excessive nor short during execution of the lock mode.
Preferably, the driving force control system includes means for detecting a vehicle speed of the vehicle, and the lock-mode engagement force-limiting means limits the lock-mode engagement force such that the lock-mode engagement force become smaller as the vehicle speed of the vehicle is higher.
According to this preferred embodiment, the lock-mode engagement force is limited to a smaller value as the vehicle speed is higher, i.e. as the main drive wheels are less prone to slip. In other words, as the main drive wheels are less prone to slip, there is less necessity of distributing the driving forces to the auxiliary drive wheels, and therefore, the lock-mode engagement force is limited to a smaller value. Thus, the clutches can be operated efficiently according to the driving forces required for driving the auxiliary drive wheels in the lock mode.
Preferably, the driving force control system further comprises driving force-calculating means for calculating a driving force of the main drive wheels, and the lock-mode engagement force-limiting means limits the lock-mode engagement force such that the lock-mode engagement force becomes smaller as the calculated driving force of the main drive wheels is smaller.
According to this preferred embodiment, the lock-mode engagement force is limited such that the lock-mode engagement force becomes smaller as the driving force of the main drive wheels is smaller, i.e. as the driving force required for driving the auxiliary drive wheels is smaller. Thus, the clutches can be operated efficiently according to the driving forces required for driving the auxiliary drive wheels in the lock mode.